In a satellite communications system, a user terminal sends signals to and receives signals from a satellite or satellites. The signals from the user terminal are relayed, by the satellite, to and from an earth station or earth stations which connects or connect with the terrestrial telephone network. In turn, those signals received by the user terminal originate with the earth station or stations, are sent to the satellite, and relayed, by the satellite, to the user terminal.
Various forms of satellite systems can be used. Where the satellite or satellites are in a geosynchronous orbit and thus orbit the earth at the same angular velocity as the rotation of the earth, the satellite or satellites do not move relative to any point on the surface of the earth, so that there is no change in the availability of satellite radio paths. For all other, non-geosynchronous, orbits, the satellite or satellites move, relative to the surface of the earth. The orbital movement of the satellites, relative to the earth, means that communications, from a user terminal, such as a mobile telephone handset, will not necessarily endure for the full period of communications. From time to time it will be necessary to switch communications from one satellite to another. One satellite will pass below the horizon while another appears above the horizon. This is not the only consideration. Atmospheric conditions and physical obstructions, local to the user terminal, can mean that the signals to and from a particular satellite are compromised even though that satellite is well above the horizon. To counter such effects, diversity operation is adopted where the user terminal is in contact with more than one satellite for the execution of a communications activity. As one signal path deteriorates, the other can be used, and vice versa.
Communications satellites can provide an area of radio coverage, on the surface of the earth, which is divided into a series of overlapping spot beams. Each spot beam can, effectively support a separate radio channel or path. The spot beams move relative to the surface of the earth. A user terminal will pass from spot beam to spot beam during a call. Handovers between spot beams must be undertaken. It is necessary to provide diversity between adjacent spot beam radio paths for all of the stated purposes in the same way it is necessary to provide diversity between satellite radio paths.
In order to establish diversity, any new satellite or spot beam, becoming available to a user terminal, must undergo an assessment to determine when it becomes a viable radio path option. In so being assessed, it is necessary for the user terminal to receive a signal from the satellite or spot beam to be assessed and report the result back to the earth station via the radio paths already established. A system for entering diversity operations is disclosed in WO97/23065 (Ericsson).
There may be many paths open to a user terminal. Because of cost and circuitry constraints, the user terminal itself may be able only to accommodate a limited number of paths within its operation, for example a maximum of two paths can be supported. When required to make an assessment of a further radio path, the user terminal must further reduce the number of communications paths for the assessment to be made on the limited resources of the user terminal. For seamless operation, with least risk of signal disruption, there is required a solution to the problem of path allocation during the assessment period.
The present invention concerns itself with the manner in which the assessments can be made, and the result reported, without any apparent interruption to any currently active, diversity communications. It seeks to overcome the problems associated with limited numbers of channels being available to a user terminal despite the potential number of channels or paths being larger. It also seeks to overcome the problem associated with selection and allocation of the limited number of channels or paths, by the user terminal, when meeting the requirement to assess other possible future paths, with least risk of communications interruption.
The present invention consists in a satellite communications system wherein a user terminal may be in radio contact with one or more earth stations through two or more radio paths to two or more satellites and wherein another satellite may potentially be included among said two or more satellites, said system being characterized by said user terminal being operative to assess which of said two or more satellites presents the highest radio path attenuation to said user terminal and to interrupt the radio path between said user terminal and that one of said two or more satellites which presents the highest path attenuation for said user terminal to assess the signal quality and suitability of said another satellite.
The invention further provides a system, wherein the user terminal is operative to select which of the radio paths between the user terminal and the two or more satellites to interrupt.
The invention further provides a system, wherein one of two earth stations providing said paths is operative to select which of the radio paths between the user terminal and the two or more satellites to interrupt.
The invention, yet further, provides a system, wherein each of the two or more satellites is operative to provide indication of its respective transmission power to the user terminal, wherein the user terminal is operative to measure the received signal from each of the two or more satellites, and wherein the radio path attenuation for each of the two or more satellites is calculated by comparing the received signal with the indication of transmitted power.
Still further, the invention provides a system wherein the user terminal is operative to provide indication to at least one of the earth stations for the selection of that radio path with most attenuation to be interrupted.
Yet further, the invention provides a system, wherein the user terminal is operative to indicate to at least one of the two or more earth stations which one of the two or more corresponding radio paths to interrupt.